Electrophotographic imaging members, e.g., photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the substantial absence of light so that electric charges are retained on its surface. Upon exposure to light, charge is generated by the photoactive or photosensitive pigment, and under applied field charge moves through the photoreceptor and the charge is dissipated.
In electrophotography, also known as xerography, electrophotographic imaging or electrostatographic imaging, the surface of an electrophotographic plate, drum, belt or the like (imaging member or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. Charge generated by the photoactive pigment move under the force of the applied field. The movement of the charge through the photoreceptor selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image. This electrostatic latent image may then be developed to form a visible image by depositing oppositely charged particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as transparency or paper. The imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite single layer containing charge photogenerating and charge transporting compounds and other materials. In addition, the imaging member may be layered. These layers can be in any order, and sometimes can be combined in a single or mixed layer.
Typical multilayered photoreceptors have at least two layers, and may include a substrate, a conductive layer, an optional charge blocking layer, an optional adhesive layer, a photogenerating layer (sometimes referred to as, and used herein interchangeably, a “charge generation layer,” “charge generating layer,” or “charge generator layer”), a charge transport layer, an optional overcoating layer and, in some belt embodiments, an anticurl backing layer. In the multilayer configuration, the active layers of the photoreceptor are the charge generating layer (CGL) and the charge transport layer (CTL).
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, however, degradation of image quality was encountered during extended cycling. The complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements, including narrow operating limits, on the imaging members. Thus, photoreceptor materials are required to exhibit, efficient charge generation and charge transport properties, and structural integrity and robustness so as to withstand mechanical abrasion during image development cycles.
Organic photosensitive pigments are widely used as photoactive components in charge generating layers. One such pigment used in the charge generating layer in electrophotographic devices is phthalocyanine (Pc). Phthalocyanines represent one of the key components of photoreceptors because of their high efficiency of charge generation. As explained, for example, in U.S. Pat. No. 5,164,493, which is hereby incorporated by reference in its entirety, polymorphism or the ability to form distinct solid state forms is well known in phthalocyanines and will affect its photoactive properties. For example, there are several titanyl phthalocyanine (TiOPc) crystal forms, or polymorphs, known to be useful in photoreceptor devices. The control of the crystal form of phthalocyanines, such as TiOPc, is critical for obtaining the desired photoactive properties, such as high photosensitivity.
Standard preparation of phthalocyanines involves synthesis at high temperature, isolation and purification, dissolution in acid followed by subsequent precipitation, and conversion to different crystal structures using organic solvents. The resulting product is expectedly in large aggregates which must be subsequently milled down to form a dispersion. The overall process results in a wide distribution of particle sizes (e.g., generally from about 100 to about 600 nm, depending on the binder and solvent system used). These wide distributions are of concern because it has been observed that the presence of large particles can cause problems such as increased charge-deficient spots (CDS) and print uniformity.
As such, processes for obtaining phthalocyanines for electrophotographic application are generally complex, multi-step processes and therefore the ability of obtaining a consistent final product with all the desired properties, including size distribution, may not be very reproducible.
The term “nanocrystal” is herein generally used interchangeably with the term “nanoparticle.”